This invention relates generally to a turntable for a record disc of the type used in reproducing sound in a high fidelity sound playback system.
High fidelity sound systems for reproducing music and voice from precision made vinyl record discs have attained a high degree of perfection. Distortion introduced by the components themselves, such as the pick-up and the electronics, are almost nonexistent. Speaker systems have also progressed to the point where they add very little undesirable characteristics of their own. The record disc itself has achieved a high degree of perfection in that the frequency response thereof is quite wide, distortion very low, dynamic range high, and surface noise very low.
Although these grooved vinyl record discs operate quite satisfactorily in present day high fidelity sound reproducing systems, one problem which has arisen relates to the fact that these discs do not always remain flat. Characteristic warping of these discs can take place either during the manufacture of the disc, during shipping or during storage. These discs may warp into a dish shape, develop saddles, or pinched areas, all of which cause some sort of up and down motion of the surface of the disc.
The stylus of the pick-up is usually cantilever mounted by a compliant suspension system from the body portion of the cartridge. A set tracking force causes a static deflection of the cantilever and stylus relative to the cartridge body. The vibration of the stylus, dictated by modulation of the record groove produces the vibration of the cantilever relative to the cartridge. This motion is amplified to be ultimately reproduced as the sound heard by the observer. The groove modulations are small and if the stylus tracks in a substantially horizontal plane the cartridge will remain at the statically deflected distance from the record disc surface. Large excursions of the stylus such as caused by warps will try to appreciably change the static deflection resulting in a change of force on the cartridge causing motion of the tone arm. Thus, a warped disc surface causes an up and down motion of the stylus other than that caused by the groove itself, and the tone arm and associated cartridge body will try to follow the stylus as it rides the warped surface.
Because the arm and the cartridge body have mass, the static deflection between the stylus and the cartridge body will necessarily change. A force vibrational situation will thereby be created between the arm, cartridge and stylus and the resonant frequency of the resulting structure will necessarily depend upon the compliance of the cartridge and the combined inertia of the arm and cartridge body. The warped disc can produce a forcing function at or near the natural frequency of the combined arm-cartridge-stylus system which will result in excessive displacement of the cartridge relative to the disc surface. This sort of unwanted vibration will cause a varying tracking force and in severe cases may cause the stylus to jump out of the groove or even cause the stylus to bottom out in its cantilever mount. Usually, these large excursions of the stylus occur at sub-audible frequencies, but modern electronic equipment and transducers can respond to such signals in a frequency range which can result in flutter of the speaker. The amplifier can thereby become overloaded, detracting from the power available for the signals which are being handled by the system.
The human ear is extremely sensitive to changes in pitch and to the rate of change in pitch. A change in pitch of the reproduced sound will occur if for any reason the velocity of the groove relative to the stylus varies from its intended velocity, as established during the cutting of the master disc from which the vinyl record disc is produced. During playback a varying platter speed, or an off-center hole in the record can both cause a mismatch between the playback and recorded velocity. Such varying platter speed can be reduced by good design of the drive system. Off-center holes in the record discs can be overcome by quality control procedures of the record manufacturer. However, the above-described tendency of vinyl record discs to warp produces a discrepency in this intended velocity of a disc with respect to the stylus which cannot be cured by present day production techniques or by design of the turntable drive system.
A warped surface of the playback disc causes the cartridge to be raised or lowered as it follows the warped surface. This action results in a pivoting of the arm, and regardless of the arm shape, or where the horizontal pivot is located, the stylus will "advance" or "return" relative to the direction of movement of a warped disc and thereby cause a change of pitch commonly known as vertical warp wow in the sound heard by the observer.
The forces which move the tone arm up and down as the stylus encounters a warped surface also causes a pivoting action of the cantilever relative to the cartridge which is not the result of groove modulation. This pivoting action takes place about a pivot which is at the vertical tracking angle from the stylus, 15.degree. to 20.degree.. Because this angle is relatively large, small deflections of the stylus relative to the cartridge body which are not caused by the groove modulations will result in the stylus "advancing" or "returning" relative to the direction of movement of the disc and thereby cause a change of pitch.
A warp can also be thought of as a cam surface which has a rise and fall. Tangents to the inflection points at the rise and fall are not level; and, therefore, if the disc is rotating at a constant angular velocity the linear velocity of the groove relative to the stylus is increased by a function of the cosine of the angle of these tangents to the level surface. With warps of short duration these angles can be quite significant. Also the surface of the record can be stretched or compressed, depending on the radius of curvature, at various points of the warped surface. This can also result in a change of pitch because the modulations are either gathered together or stretched farther apart.
All these various forms of pitch change due to record warp can occur simultaneously. In some cases the deleterious effects will complement one another leading to readily detectable pitch changes, and in other cases might tend to cancel one another depending upon the type of warp and the geometry of the tone arm and the dynamics of the system. Depending upon the type of tone arm used in a particular turntable structure the up and down motion of the tone arm might also result in corresponding "in" and "out" motion of the tone arm relative to the center of the disc. This sort of motion will cause the tracking force on the side walls of the record groove to vary, thereby to a sideward swaying of the tone arm and possible distortion of the sound produced by the system.
The chief aim of the present invention is to provide a turntable that flattens the warped record disc while the record disc is being played in a present day high fidelity sound reproduction system. This flattening of the record disc is accomplished by applying vacuum to the underside of the disc while the disc is supported in the area opposite that portion of the record being engaged by the stylus. More particularly, the record is supported on an uninterrupted flat surface defined by a platen which preferably comprises an annular plate glass segment supported in the turntable structure for rotation, and provided with evacuated areas adjacent its inner and outer peripheries.
Another advantage of the present invention is in the provision of a rigid back-up surface for the record disc being played such that "needle talk" is reduced. The reduction in "needle talk" is achieved as a result of supporting the record disc uniformly across its entire underside rather than merely supporting the disc adjacent its periphery or in certain limited areas such that the vinyl record disc tends to act as a sounding board or the like which interferes with transferring the groove modulations to the pick-up stylus. As a result of firmly anchoring the record disc against the rigid and comparatively massive platen a "cleaner" and less distorted sound is produced in a system incorporating a turntable constructed in accordance with the present invention.
Still another advantage of the present invention is the elimination of the complaint elastomeric surface sometimes utilized in turntable platters which surfaces cannot support a warped record in a flat condition because of the fact that the resiliency of the record itself will cause distortions in the underlying complaint supporting surface of the platter. Although the application of a vacuum to such a complaint or elastomeric platter surface will yield a certain degree of flattening in the disc, it is an important feature of the present invention that the disc is absolutely flattened against the rigid, flat platen surface in such a way that the only variation to the flatness of the upper surface of the record disc is caused by variations in the thickness along circumferential lines of the record disc itself. Needless to say, these sort of variations in the flatness of the upper surface of the record disc can be made insignificant by proper production control of the record manufacturer.
A still further advantage of the present invention is the provision of a flat uninterrupted surface of the platen which supports the record disc such that no cavities or holes are provided in the surface, and yet a vacuum is provided for drawing the record disc against the platen surface to flatten out the record disc and thereby eliminate any warping thereof. As a result of the lack of holes or pores or the like in the platen surface, it will be apparent that the surface is more readily cleaned than would be the cause with a porous or partially compliant platter surface of the type available heretofore.